This invention relates to process control apparatus, and more particularly, to an automatic, pneumatic controller of a process variable such as process pressure.
Prior to this invention, several automatic, pneumatic controllers have been disclosed and commercially used. U.S. Pat. No. 3,572,360 issued Mar. 23, 1971 to S. G. Lloyd et al. (incorporated by reference) discloses a prior pneumatic controller of Fisher Controls Co., Inc., while the following patents disclose controllers of others: U.S. Pat. No. 3,515,162 issued June 2, 1970 to H. L. Bowditch et al.; and U.S. Pat. No. 3,047,002 issued July 31, 1962 to H. R. Jaquith. While each of these controllers has proven desirable in its time, significant problems have been common to the art. A major source of complication in the design of process controllers has been the provision of set point and gain adjustment by mechanisms which do not interact with each other and thereby produce artificial disturbances to the process. The provision of indicators of the set point and process variable, which must operate in the process environment, has put additional burdens on controller design. In the past, two known methods have been employed to reduce or remove the interactions that can exist between the input or process variable adjustment mechanism, the gain or proportional band adjustment mechanism, the set point adjustment mechanism, the set point indicator mechanism and the input indicator mechanism. First, interaction has often been minimized by the application of levers, spring rates, and friction points where motions or forces have been summed. This method has often required intricate adjustment and maintenance procedures and has been adversely susceptible to external vibrations. The second method has been the use of pneumatic amplifier systems, which have operated mechanical elements while producing no perceptible load or interactions to their input elements. This style of construction has been commonly used in controllers in control room environments, often referred to as receiver controllers. This method of "pneumatic coupling" has often required extensive development to assure that static and dynamic characteristics could be maintained for all control contingencies. Additional consumption of supply air has also been a negative aspect of this method. The design complexity and maintenance difficulty of either method has resulted in a high purchase price of controllers, and maintenance difficulty.